An important issue arose in designing a structure of a light-emitting diode (LED) is to evenly spread current from bonding pad to p-n junction in order to reach a better light-emitting efficiency. The known technologies such as semiconductor window layer, transparent conductive oxide film, and patterned electrode, are already used to boost the current-spreading performance.
GaP window layer is usually adopted in AlGaInP series LED. GaP has an energy band gap (Eg) of 2.26 eV, and is transparent to red, orange, yellow light, and part of green light spectrum, and is an indirect band gap semiconductor, which absorbs less light in comparison with a direct band gap semiconductor. A GaP layer of a sufficient thickness such as 2 μm˜30 μm exhibits an acceptable current-spreading behavior, and the thicker the GaP window layer is, the better the current-spreading performance can be achieved. However, it takes much time to grow a thicker window layer and the throughput is reduced.
The transparent conductive oxides such as ITO, CTO, and InO are also used to enhance the current-spreading performance. For instance, ITO has a 90% transmittance in the range of 500 nm˜800 nm in wavelength with a resistivity of 3×10−4 Ω-cm and a sheet resistance of 10Ω/□. Generally speaking, a small scale chip with an ITO layer of 0.1 μm˜1 μm can acquire an acceptable current-spreading outcome. The required thickness can be formed in a short time by using the known manufacturing method such as sputtering and electron beam evaporation. However, the ITO also falls short of the current-spreading requirement in response to the increasing area of light-emitting diode chip (for example, chip size≧15×15 mil) and the development of rectangular chip.
Patterned electrode is another way that is often used to elevate current-spreading performance. In the method, to evenly spread current from the patterned electrode to p-n junction, the electrode is arranged by outwardly extending from a connection, interdigitating p- and n-electrodes, or forming as dots, mesh or other patterns. To form a patterned electrode, extra quantity of electrode material is often needed in order to cover larger area of the upper surface of the light-emitting diode. Moreover, the material adopted in the patterned electrode is usually an opaque metal, and consequently the light-emitting efficiency is greatly degraded.